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Loud thumping / banging when zone turns on...

To give some background, we recently had an plumber out as we had water hammer whenever our sprinklers would turn off (we had no issue with them turning them on, just the hammer when they would turn off). The plumber found two issues: 1) the pipes weren't secure, so were moving around quite a bit with the force of the water turning off, and 2) our expansion tank needed to be replaced. The plumber secured the pipes and replaced the expansion tank. All seemed to be quiet and working properly!

Fast forward to 3 days later. Our sprinklers came on and everything seemed to be working well...until zone 2 came on. When zone 2 turns on, there was a loud bang and then a loud thumping for a about 7 seconds or so (seemed like forever) until the system filled and the water flowing out of the sprinklers. No problems with any of the other zones. When I ran the system again about an hour later, no bang or thumping and all seemed to be working. It's been 3 weeks now, and every time zone 2 comes on, I get the same issue. It is the worst whenever zone 2 comes on for the first time after not using it for a while. We have 7 zones total, and this only happens on zone 2 - the other zones are smooth (in fact, you can barely tell they are on). We had the plumber come back out and he thinks it is the valve in zone 2.

Since this is only occurring on zone 2, my guess it that it might be a combination of our sprinkler system and the solution found in the link above. It is possible that zone 2 is somehow losing more pressure than the rest of the zones in-between cycles causing this issue? The pressure fix makes sense to me, but if that was the case, I would think there's be a problem on all of the zones. My thought is maybe some kind of leak that is causing the pressure drop in zone 2, but I'm kind of out of ideas (we're new homeowners so still learning about how all of this works!)

Sorry that this post is so long, but any ideas would be greatly appreciated!

Thanks for the responses. I'm pretty sure it's when zone 2 turns on. I stood down in our basement during the switch as I thought it was a problem with the shutoff of zone 1. Zone 1 shuts off and there's very little noise. About a second later, zone 2 comes on and as soon as the water starts flowing to zone 2, it makes the thumping noise (and can last quite a few seconds). I'll check both though a the controller to see. I'll also see if there's a way to tone down the flow of zone 2.

Well, apparently our valves don't have flow control, so I guess we'll have to look into other solutions. Maybe replace the valve or put a PRV before the sprinkler system. If there are any other ideas, I'd greatly appreciate it. Thanks again for all of the help so far!

"there was a loud bang and then a loud thumping for a about 7 seconds or
so (seemed like forever) until the system filled and the water flowing
out of the sprinklers."
" It is the worst whenever zone 2 comes on for the first time after not using it for a while"
"...about an hour later, no bang or thumping..."

Is your yard sloped? Could the water in the zone be draining out through the low head leaving most of the zone empty?
The water rushing to fill the pipes might explain what you hear. It can take hours for this to happen so turning it on an hour later meant some water would still be in the pipes.

Yes, we are on a slope, so that's definitely possible. I don't ever remember this happening when we turn the sprinklers on for the first time when the season start (the system is drained for the cold winters).

Is it possible that since the expansion tank was replaced there's somehow a change in pressure to the sprinkler system that could be causing this and lowering the pressure would fix the problem?

We have two PRV's attached to the system. One is before the water meter (our meter happens to be in our basement - not sure why they did it this way) and one after the meter and after the cutoff to the sprinkler system, but before the pipe leading to the rest of the house. Since we didn't hear this banging before, my only guess is that it has something to do with the expansion tank (the plumber didn't touch anything else, at least as far as I know).

Since a PRV greatly inhibits backwards flow, a pressure tank downstream shouldn't make a difference. If the PRV was stuck open (not likely) then the pressure tank would come into play.

The easiest no-plumbing-required fix you can try is the flow control bonnet. Once a zone valve is throttled down, water can't rush through it like in a standard valve, and that can be enough to stop the banging.

Thanks again for your help. One thing I did notice is that since the new pressure tank was installed, the water pressure to the sprinkler system seems to be higher. I did a "test" last night by turning the system on zone 1 and let it run until right before the switch to zone 2. Before the switch, I turned a few faucets on and ran the water inside the house while it switched from zone 1 to zone 2. No banging, thumping, or loud noises of any kind (and I hadn't run the sprinkler system since the day before, so it should have made some noise). It seems that if water is running in the house, it doesn't have the problem. I'm going to check again tonight to verify that is the case.

My thoughts are the same as yours - throttle down the pressure to zone 2 with a flow control bonnet. I'm thinking that the alternative is to check pressure before the PRV to the house to see what the incoming water pressure is. Since we have 2 PRV's (one before the water meter, and then one after the sprinkler cutoff but before the rest of the house) I can adjust the incoming water pressure via that first valve. Is there anything wrong with this approach? I'm not sure what the standard pressure range should be on the incoming line.